Until recently, the majority of vaccine efforts directed against HIV-1 have targeted the structural proteins. These efforts are rooted in the empirical nature of vaccine development and prior successes with other viral pathogens. In this proposal, we provide evidence that an HIV-1 regulatory protein, namely Tat, is an excellent vaccine target based on its immunosuppressive properties. We provide evidence that anti-Tat antibody correlates inversely with p24 antigenemia and furthermore, that anti-Tat antibody is a good prognostic indicator for clinical stability. In addition to the role of Tat as a viral transactivator, HIV-1 Tat primes non-infected T cells for activation induced apoptosis. Tat acts extracellularly to inhibit proliferation of T cells in response to recall antigens and enhances the permissivity of non-infected primary cells to HIV infection. Tat therefore is critical for viral replication and dissemination and plays an important role in viral induced immune suppression. To develop a vaccine against Tat, which can be considered an immune toxin, we have developed methodologies for large scale purification and chemical modification, producing an inactive, yet immunogenic protein, "Tat Toxoid." We have recently demonstrated the safety and potency of this vaccine approach in a Phase I clinical trial in 14 HIV infected individuals. In this "therapeutic" vaccine trial, a statistically significant increase in CD4+ cells was observed along with a trend toward a reduction in viral loads. The ultimate goal of this approach will be to develop effective "protective" vaccine strategies for seronegative individuals. Numerous difficulties are inherent in protective HIV-1 vaccine trials in humans, namely, the large number of volunteer participants required. To reduce this number, patients at high risk for HIV-1 infection can be considered, however, ethical considerations mandate counseling regarding behavioral risks for HIV-1 infection, adding to the complexity of such endeavors. In order to develop an effective protective vaccine strategy suitable for human testing, we believe that animal models for vaccination and virus challenge should be utilized. In order to test the efficacy of Tat Toxoid as a protective vaccine component, we propose to immunize rhesus macaques and challenge these animals with a chimeric human/simian immunodeficiency virus (SHIV) containing the HIV-1 Vpu, Envelope, Tat, and Rev proteins. The SHIV isolate, 89.6P, induces CD4+ cell depletion and AIDS like illness in macaques. Our proposed studies will determine the safety, immunogenicity, and efficacy of the Tat Toxoid vaccination and determine the effect of Tat neutralization in enhancing humor and cellular immunity to other viral antigens (i.e. Envelope). The epitope specificity of anti-Tat antibodies produced in rhesus macaques will be determined and compared to neutralization activity in biologic assays.